In the science of ballistics, it is conventional wisdom that shock waves, emanating from a projectile in flight and traveling faster than the speed of sound, interfere with and break-up the flow of air close to the aft or back end of the projectile. The disruption of air flow affects the flight of the projectile. In order to compensate for or overcome such perceived interference and to impart spin to a projectile fired from a non-rifled or smooth bore system, the projectile 120, as shown in prior art FIG. 1a, is manufactured to include a boom or extension 122 which provides distance between the nose 124 and fins 126. In effect, the boom ensures that the fins, which do not extend beyond the diameter of the body of the projectile, will contact intact air flow.
Alternatively, a projectile, as shown in FIG. 1b may have expanding fins 128. In such a case, the fins are hinged and spring loaded to the body of the projectile so that as the projectile exits the bore of a cannon on firing, fins 128 expand beyond the caliber or diameter of the body of the projectile to engage intact air flow causing the projectile to spin.
The prior art projectiles may have an ogive or rounded nose 124 as shown in FIGS. 1a and 1b or a spine nose discussed infra.
The structures described above add expense to the manufacture of the projectile and may require movable parts that are subject to failure, as in the case of the expanding fins of FIG. 1b.
It has now been found that the device of the present invention, having a diameter no greater than the diameter of the projectile to which it is attached, and which can be attached at the aft end of the projectile, can successfully use the air flow near the aft end of the projectile to spin the projectile. Accordingly, the invention eliminates the need for a "boom" or expandable spring loaded fins.